United States Patent (15) 3,658,510 Hoffmann et al. (45) Apr. 25, 1972

(54) RECOVERY OF SILVER FROM 712,640 1 1/1902 Betts...... 75/99 ELECTROLYTC COPPER REFINERY 2,150,366 3/1939 Ehrhart. ...75/1 12 X SLIMES 2,060,539 1 1/1936 Spies...... 75/18 X 1,448,475 3/1923 Weisberg...... 75/118 (72) Inventors: James E. Hoffmann, Plainfield; Runyon G. 1,315,660 9/1919 Ferguson...... 75/99 Ernst, Woodbridge, both of N.J. 73) Assignee: American Metal Climax, Inc., New York, Primary Examiner-L. Dewayne Rutledge N.Y. Assistant Examiner-G. T. Ozaki Attorney-Kasper T. Serijan and Eugene J. Kalil 22 Filed: Apr. 14, 1970 (21) Appl. No.: 28,370 57) ABSTRACT Silver is recovered from electrolytic copper refinery slimes 52) U.S.C...... 75/99, 75/101 R, 75/120, containing, in addition to silver, one or more of the elements 75/114, 75/117, 75/118, 75/119, 75/121 Se, Te, Pb, Sb, Sn, As, Bi, Zn, Cu, Au, Ni. Fe, among others. A 51 Int. Cl...... C22b. 15/08 slurry is formed of the slimes in a hydrochloric acid solution 58) Field of Search...... 75/99, l 18, 100, 1 12, l 14, which is agitated, during which excess chlorine gas is added to solubilize substantially all of the elements present, the silver 75/120, 101 R; 23/98, 87 being substantially quantitatively converted to silver chloride (56) References Cited to form a silver-enriched residue. The residue is thereafter separated from the solution and the silver then recovered from UNITED STATES PATENTS the residue. 467, 17 l l 11892 Hoyt...... 75/114 9 Claims, 2 Drawing Figures

3,658,510 1. 2 RECOVERY OF SILVER FROMELECTROLYTIC from the separated residue containing most of the other ele COPPER REFENERY SLIMES ments which are accessible to recovery without using pyrometallurgical techniques. This invention relates to the recovery of silver from elec A still further object is to provide a method for recovering trolytic copper refinery slimes containing selenium and/or tel high purity silver at high yield levels from electrolytic copper lurium, in addition to other elements, depending upon the refinery slimes, the silver being hydrometallurgically source of the copper from which the electrolytic slimes are recovered in the form of high purity silver powder. derived. Another object of this invention is to eliminate the necessity Most electrolytic copper refinery slimes contain precious of using the Dore furnace cycle with its attendant energy and metals in which silver generally predominates. In recovering 10 reagent requirements, along with long processing times, thus the precious metals and, in particular, silver, selenium and/or freeing the furnace for other service. tellurium are usually obtained as a byproduct. Electrolytic These and other objects will more clearly appear from the copper refinery slimes are often called anode muds, elec following disclosure and the appended drawing, wherein: trolytic residues, sludges, and the like, which are all meant to FIG. 1 is a flow sheet showing the steps employed in be included in the term "slimes' as used herein. It is not un 15 recovering silver as high purity silver powder from electrolytic common for such slimes to contain Se, Te, Pb, Sb, Sn, As, Bi, copper refinery slimes; and Zn, Cu, Ag, Au, Pt, Pd and Fe, among other elements. FIG. 2 illustrates the quantitative breakdown of the BACKGROUND OF THE INVENTION AND THE PRIOR recovery starting with 2 kilograms of slimes having an analysis ART 20 of 21.5% by weight of silver. One prior art method of slimes treatment with the objective SUMMARY OF THE INVENTION of recovering silver as one of the major byproducts has been to Stating it broadly, a hydrometallurgical method is provided first screen out large particles of metallic copper and then for recovering silver from electrolytic copper refinery slimes decopperize the slimes of the bulk of the remaining copper by 25 which contain, in addition to silver, one or more of the ele subjecting the slimes to an acid leach, e.g. sulfuric acid leach, ments Se, Te, Pb, Sb, Sn, As, Bi, Zn, Cu, Ni, Fe, Au, and the preliminary to further treatment. The slimes thus treated are platinum group metals, such as Pt and Pd, among other ele then smelted in a Dore furnace, during which some of the con ments. The method comprises forming a slurry of the slimes in tained selenium is volatilized and partially recovered via a hydrochloric acid solution which is agitated while being scrubbers as a solution. During the smelting operation, the 30 simultaneously chlorinated with chlorine gas. During the molten bath is subjected to three slagging steps comprising a chlorination step, substantially all of the elements are solubil first slagging with silica to take up mold wash, siliceous gan ized with the exception of silver and one or more of the lead, gue, Al,Oa, etc. The silica slag is then replaced with a soda ash antimony, tin or silica that might be present, whereby a silver slag (e.g. Na,CO) which oxidizes any selenides present to rich residue is obtained containing silver chloride, with or SeO gas for flue recovery, although some of the selenium may 35 without the aforementioned contaminants. The silver-rich form selenates which enter the soda ash slag together with tel residue is separated from the solution, which solution may be lurites. This slag is skimmed off and replaced by a niter slag subsequently treated for the recovery of the remaining pre (NaNoa) which oxidizes out other elements, such as Sb, Pb, cious metals, Se, Te, etc. The silver-rich residue is subjected to Cu, small amounts of Se and/or Te, etc., which enter the slag. hot water leaching (e.g. acidified water) to remove the bulk of The molten metal remaining in the Dore furnace is rich in 40 the lead chloride present, thereby decreasing subsequent am silver. A typical analysis of the Dore metal may comprise at monia requirements and enhancing the leaching of the silver. least about 95% of Ag, up to about 4% of Au, Pt and Pd and The leached residue is further leached with ammonium residual copper in amounts less than 1%, e.g. ranging desirably hydroxide to solubilize the silver chloride. The hot water from about 0.1 to 0.5% by weight. The metal is cast into a leaching enhances the leachability of the silver in ammonium plate which is then transferred to an electrolytic parting plant 45 hydroxide and aids in reducing the ammonia requirements. where the silver is electrolytically dissolved out of the Dore' The solution with the dissolved silver is separated from the plate employed as an anode in an electrolytic cell to produce residue and the residue washed, the wash water being col high purity dendritic silver (99.99%) at a graphite cathode at lected with the solution, which is thereafter heated to the bottom of the cell, a sludge being formed as a by-product evaporate the ammonia and produce a very pure silver containing substantially all of the precious metals Au, Pt and 50 chloride precipitate. The precipitate is then reduced to silver Pd which sludge is subjected to further recovery treatments. powder in an alkaline solution using any of a variety of organic An important problem characteristic of Dore furnacing is the materials suitable as reducing agents therefor, which include fact that a substantial amount of silver enters the slag and must but are not limited to molasses, formaldehyde, acetaldehyde, be recycled, thereby increasing "run-around' time. sucrose, dextrose, among other sugars, especially the reducing The foregoing method is somewhat expensive in view of the 55 sugars, and the like. necessity of pretreating the slimes using pyrometallurgical techniques. Moreover, the method is not wholly efficient. DETAILASPECTS OF THE INVENTION Since silver is the major metal being recovered, it would be The invention will now be described in detail with respect to desirable to provide a method in which substantially all of the 60 FIGS. 1 and 2 as follows: silver can be easily and quickly separated from substantially With regard to FIG. 1, reference is made to Step 1 in which all of the remaining elements without requiring an elaborate copper refinery slimes containing about 21.5% silver are pretreatment of the slimes and without the necessity of using suspended in a solution of hydrochloric acid in reactor 10, the rather involved pyrometallurgical techniques which are strength of the acid being dependent upon the subsequent known to be expensive and not always efficient. 65 treatments to be employed. An advantageous feature of the in OBJECTS OF THE INVENTION vention is that a relatively high solids loading can be employed in producing the slurried suspension, for example, a solids It is thus the object of the invention to provide a loading of 1 kg per liter of solution. The suspension is hydrometallurgical method of directly separating silver from vigorously agitated as indicated in Step 1 and the solution electrolytic copper refinery slimes. 70 chlorinated with gaseous chlorine at the maximum rate for Another object is to provide a hydrometallurgical method rapid conversion without significant chlorine gas of separating silver from copper refinery slimes in which a breakthrough. A plurality of reactors may be used in series in silver-rich residue is produced in a first step having the silver which the excess exhaust chlorine gas from one reactor can be quantitatively retained therein and from which the silver is introduced into the next one, thereby assuring 100% chlorine subsequently quantitatively recovered, the filtrate remaining 75 consumption efficiency. 3,658,510 3 4. The reaction is sufficiently exothermic so that the heat ric amount necessary to convert the silver chloride to silver liberated raises the mixture to the boiling point (e.g. 100 to hydroxide as follows: AgCl-NaOH - NaCl-i-AgOH plus an l 12° C) and, if necessary, the mixture is maintained at the additional amount required to provide an alkaline solution of boiling point by the addition of heat. Preferably, Step 1 is car approximately 1 Normal after conversion of the silver ried out under reflux to maintain as high an acidity as possible chloride. After the silver chloride has been converted to silver in the solution. Hydrochloric acid is generated in the chlorina hydroxide, a concentrated solution of dextrose or other reduc tion step; however, if the system is run without reflux, it tends ing sugars is added to the reactor while the solution is continu towards an azeotropic composition at s 6 normal concentra ously agitated. The reducing sugars convert the silver oxide to tion. A cold reflux condenser at about O' to 10 Cenables the pure silver metal powder which is filtered off at 21 and reaction to be completed at a higher acid concentration, for 10 washed, the dry silver particles being then melted in a crucible example, at an acid concentration of about 8 to 9 normal or 22 (Step 6) e.g. a graphite crucible, and cast into silver ingots higher, e.g. 12N. 23. The final acid concentration in the subsequent treatment Care must be taken in reducing the silver oxide (AgO) with steps will depend upon the initial acidity and the quantity of dextrose as the reaction is highly exothermic according to the chloride formers present which vary greatly with the slime. 15 following reaction: Normalities as high as about 15 have been obtained on occa 12 AgO + CHOs - 6CO, + 6HO -- 24 Ag. sion with the reflux temperature. The rate at which the dextrose is added should therefore be Upon completion of the chlorination treatment, the residue controlled. Any number of other organic substances may be which is now enriched in silver as silver chloride and, possibly employed to reduce the silver oxide. For example, KARO containing some lead chloride, is filtered at 11 to form a fil 20 syrup may be used. Examples of other organic reducing agents trate 12 containing Se, Te, precious metals other than silver, are recited hereinbefore by way of illustration and not by way among other elements, which filtrate is then set aside, and the of limitation. chlorination residue then washed, the wash water being thereafter combined with filtrate 12. The washed chlorinated The melting of silver in Step 6 is carried out under oxidizing residue is then leached in Step 2 with acidified boiling water 25 conditions to insure burning off of any entrained organics, foll (e.g. pH of about 3) by agitation in vessel 13 at a solids-liquid lowed by reducing and cooling under reducing conditions. ratio of about 10:1 based on dry solids weight to remove the An advantage of the foregoing process is that it enables a bulk of the lead chloride present. Generally speaking, the pH substantially quantitative recovery of silver from electrolytic of the acid leach may range from about 0.5 to 5, it being un copper refinery slimes using a relatively simple hydrometallur derstood that the pH need not necessarily be limited to this 30 gical technique over a relatively low temperature range in the range. Leaching is carried out hot under agitation for about 1 neighborhood of about 110° C. As illustrative of the quantita hour, such as at the boiling point. The leached slurry is settled tive yields possible in going from Step 1 to Step 6, the follow using a flocculant, the leach liquor being then decanted or fil ing specific example is given with reference to FIG. 2. tered and the solids thereafter filtered at 14 and washed with EXAMPLE hot or boiling water. 35 As stated above, the function of the hot acid leach and the A total of 2 kilograms of electrolytic copper refinery slimes hot water wash is to remove the bulk of the lead chloride were slurried in 2 liters of 5N HCl. Unless otherwise stated, all present in the chlorination residue. The removal of lead percentages are by weight. The slimes analyzed about 21.1% chloride serves to reduce the amount of ammonia consump 40 silver, showing a silver content in the slimes of about 422 tion during the subsequent ammoniacal leach. It also serves to grams (note box 25 of FIG. 2). The slurry was agitated while increase the % extraction of AgGl from the chlorination chlorine gas was sparged into it, the temperature rising about residue during the ammonia leach. The lead value in the leach 100° C and the temperature maintained at substantially that liquor can be recovered easily by neutralization of the liquor level by the addition of heat whenever necessary. A with soda ash (Na,CO). No significant amount of silver dis 45 chlorinated residue was obtained (box 26) amounting to solves in the lead chloride leaching step. about 1.412 kg and assaying 29.7% silver. The residue was fil The water leach chlorination residue is then subjected to tered to provide a chlorination filtrate 27 which is combined Step 3 by preferably forming a slurry of the residue in water in with the water leach 28 and set aside for recovery of such solu vessel 15 and adding concentrated ammonia to it. Generally bilized elements as Se, Te, Pt, Pd, Cu, Ni, etc., originally speaking, the concentration of ammonia is rather flexible and 50 present in the slimes, the filtrate also containing a very small may range from about 2N to 15 N.The ammonium require amount of silver which is recoverable and, therefore, not lost. ments are calculated on the basis of 1 to 1.5 grams of am An amount of 50 grams was removed from the 1.412 kg of monia per gram of residue taken on the dry basis. The am residue for analysis corresponding to 14.9 grams Ag. The monia leaching is carried out for about 1 hour cold, for exam silver, along with some lead, antimony, tin and silica, ple, at ordinary temperature. The leached slurry is then fil 55 remained behind in the residue (26). The silver distribution as tered and washed at 16 and the washings and filtrate com between the chlorination residue and the chlorination liquor bined, the residue being cycled to the smelter for recovery of (27) was as follows: residuals, The combined ammonia filtrate and washings from the am Chlorination Residue 99.7% monia leach are treated according to Step 4 in closed reactor 60 Chlorination Liquor 0.3% 17 and heated while agitated to drive off the ammonia which is subsequently recovered. Silver chloride is produced as a The chlorination residue, after drying, was leached in boil suspension according to the following reaction: ing acidified water (28) in order to remove the lead chloride Ag(NH3)C - AgCl+ 2 NHat. present. The acidified water contained one gram of HCl per The distillation is terminated when the boiling point reaches 65 liter of solution. The leaching is carried out at about 100°C 100 C, indicating removal of virtually all of the ammonia as a for 1 hour at a ratio of solids to liquid of about 1:10 by weight. gas. The silver chloride is filtered at 18 and the filtrate 19 set The suspended leached solids were settled with a flocculant aside, the silver chloride precipitate being then treated ac and subsequently filtered off (29) to provide a residue (31), cording to Step 5. An alternative to boiling off the ammonia is the filtrate (30) being removed and set aside. to neutralize the solution with sulfuric or nitric acid and 70 The water leach residue (31) weighed 1.054 kg and con precipitate the silver chloride. tained 39.6% silver. The silver was analyzed by removing 33 In recovering silver under Step 5, the silver chloride, which grams (32), leaving a total of 1.021 kg for the ammonia leach is of very high purity, is slurried in reactor 20 in a hot (33). The silver distribution between the solids (31) and the vigorously agitated solution of sodium hydroxide. The sodium filtrate (30) was as follows: hydroxide requirements are based on at least the stoichiomet 75 3,658,510 5 6 % Ag in solids 99.9% % Ag in filtrate 0.1% lectively separated from the nonvolatiles, such as silver. The disadvantages inherent in this system are the problems related The hot water leach residue containing approximately 40% to the materials of construction arising from the corrosive ef silver was then leached with ammonia under the following fects of the molten salt bath, the difficulties encountered in conditions: mechanically agitating the fused mixture and the additional problem of maintaining a tight seal at elevated temperatures. Grams NH/gram residue The advantages of the wet chlorination process are that Ammonia conc. 15% silver is directly recovered without using pyrometallurgical Time 1 hour 10 techniques, the maximum temperature need not exceed 110 After completion of the leaching stp, the insoluble residue C; the rise in temperature is limited to the boiling points of the was filtered (34) and washed with a 5% ammonia solution, aqueous solutions used; silver and the bulk of the lead present leaving a NH3 leach residue of 456.5 grams (35). The 5% NH are left in the residue in a one-step treatment operation, while wash solution is combined with the NHaleach filtrate, prior to the remaining elements, e.g. Se, Te, Sb, As, Sn, Cu, Ni, and distillation, which was distilled (36) to remove the ammonia 15 the precious metals are simultaneously solubilized; no charge and to precipitate silver chloride which was filtered at 37 and preparation is involved as in the two patents mentioned above, a highly pure silver chloride residue (38) obtained weighing and the reaction rates are relatively fast. A particularly impor 528.4 grams and containing about 74% silver. tant advantage is that silver is separated from the remaining The distribution of the silver between the ammonia leach precious metals in the first step, thereby eliminating the need liquor and leach residue demonstrates that substantially all of 20 for a parting plant. the silver originally in the slimes was chlorinated. Although the present invention has been described in con junction with preferred embodiments, it is to be understood Ammoniacal filtrate 98.94% silver that modifications and variations may be resorted to without Ammonia each residue 1.06% silver departing from the spirit and scope of the invention as those 25 skilled in the art will readily understand. Such modifications In actual practice, the ammonia leach residue (35) is and variations are considered to be within the purview and returned to the smelter for recovery of residual silver and scope of the invention and the appended claims. trace precious metals. What is claimed is: The silver chloride precipitate (38) obtained after the am 1. In a method of separating silver value from electrolytic monia distillation was slurried in a hot aqueous solution (39) 30 copper refinery slimes containing elements of the group con containing 114 grams per liter of sodium hydroxide (alkali sisting of Se, Te, Pb, Sb, Sn, As, Bi, Zn, Cu, Ni, Fe, Au, and the metal hydroxide), the silver chloride concentration being platinum group metals, the improvement which comprises, about 265 grams per liter. The AgCl reacted with the sodium forming a slurry of said slimes in a hydrochloric acid solu hydroxide as follows: tion, : AgCL + NaOH - AgOH 35 agitating said slurry while chlorinating said solution with 2 AgOH - AgO + H2O gaseous chlorine whereby to solubilize substantially all of The silver oxide in the slurry was then reduced to silver by the elements in said slimes, while converting said silver to the addition of dextrose in which the end products are very an insoluble silver chloride precipitate and form a silver high purity silver powder, CO, and H2O. The endpoint of the 40 enriched residue, reaction is determined when the color of the insoluble phase and then separating said silver-enriched residue from said changes from a black amorphous solid to a silvery colored solution containing said solubilized elements. dense precipitate. The distribution of the silver after filtering 2. The method of claim 1, wherein the slurry is formed in a (40) between the silver powder (42) and the spent reduction hydrochloric acid solution having a normality ranging up to liquor (41) is given as follows: 45 about 12N. % Ag reporting as Ag powder 99.999% 3. The method of claim 1, wherein the silver-enriched % Agloss in final solution 0.00015% residue is washed with acidified boiling water to remove the bulk of lead chloride present in the residue. The silver recovered as silver powder 42 came to 388 4. A method of separating silver value from electrolytic grams, while the silver removed during the sampling for analy 50 refinery slimes containing elements selected from the group sis amounted to 28 grams, thus resulting in a total of 416 consisting of Se, Te, Pb, Sb, Sn, As, Bi, Zn, Cu, Ni, Fe, Au, and grams. This accounts for 99% of the total silver (422 grams) in the platinum group metals, which comprises, the slime, the 2.8% being recycled back into the process via forming a slurry of said slimes in a hydrochloric acid solu filtrates and residues. As is apparent, the silver is substantially tion, quantitatively recovered from the slimes. 55 agitating said slurry while chlorinating said solution with The foregoing process is an improvement over the methods gaseous chlorine, thereby solubilizing substantially all of disclosed in U.S. Pat. No. 3,249,399 and No. 3,288,561. In the the elements in said slimes, while converting said silver to dry chlorination process of U.S. Pat. No. 3,249,399, the silver insoluble silver chloride and forming a silver-enriched and precious metals plus Cu, Ni, Pb are left in the residue 60 residue, separating the residue from said solution contain while Se, Te, Sb, As and Sn are volatilized, the chlorination ing said solubilized elements, temperature ranging from about 300°C to 500°C. The disad leaching said residue with acidified boiling water in order to vantages of this process are that relatively high temperatures remove the bulk of any lead chloride present in the are employed which are difficult to control. In addition, the residue, slimes must be specially treated by agglomerating the slimes in 65 further leaching said leached residue with an ammoniacal admixture with an inert binder, following which the ag solution to form soluble silver ammonium chloride, glomerated slimes are contacted by dry chlorine to convert at separating said solution as a filtrate from the residue, least a major portion of Se, Te, Sb, As, and Sn into volatile and quantitatively recovering said silver value from the fil chlorides. Thus, silver, which is left in the residue, is recovered trate. at the end of the process. 70 5. The method of claim 4, wherein the slurry is formed in In U.S. Pat. No. 3,288,561, the slimes are mixed with alkali the hydrochloric acid solution having a normality ranging up metal halides and the mixture formed into a fused salt bath, to about 12N. through which gaseous chlorine is bubbled at temperatures 6. The method of claim 5, wherein the silver is quantitative ranging from about 350° C to 900C, whereby the Se, Te, Sb, ly recovered as high purity silver chloride by neutralizing the As and Sn values are converted to volatile chlorides and col 75 ammonia with an acid. 3,658,510 7 8 7. The method of claim 5, wherein the silver is quantitative- 9. The method of claim 8, wherein the silver chloride is ly recovered from the ammoniacal solution as high purity reduced to metallic silver by adding the silver chloride to an silver chloride by distilling the ammonia. alkaline solution formed of an alkali metal hydroxide contain 8. The method of claim 7, wherein the silver chloride is ing a reducing agent therefor. reduced to metallic silver. 5 x xk six k sk

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